drm/amd/display: Check phantom_stream before it is used

commit 3718a619 upstream.

dcn32_enable_phantom_stream can return null, so returned value
must be checked before used.

This fixes 1 NULL_RETURNS issue reported by Coverity.

Reviewed-by: Rodrigo Siqueira <rodrigo.siqueira@amd.com>
Signed-off-by: Jerry Zuo <jerry.zuo@amd.com>
Signed-off-by: Alex Hung <alex.hung@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
[Xiangyu: BP to fix CVE: CVE-2024-49897, modified the source path]
Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>